Issue 62

Y. S. Rao et alii, Frattura ed Integrità Strutturale, 62 (2022) 240-260; DOI: 10.3221/IGF-ESIS.62.17

Microhardness In the present study, the Vicker’s hardness of the composite laminates was studied by applying a 100 g force for 20 seconds on a pyramid-shaped diamond tip indenter placed over the composite surface. Vicker’s microhardness versus filler concentration in BN-CFREC and MoS 2 -CFREC is plotted in line graph shown in Fig. 10. The microhardness increased with the hBN and MoS 2 fillers upto 6 wt.%. This is attributed that the hBN and MoS 2 uniform dispersion in the matrix alters the cross-linking structure and improved its resistance to deformation through contribution of intrinsic strength and modulus of fillers to the matrix. It is noticed that the microhardness of BN-CFREC is slightly higher than MoS 2 -CFREC due to the more stiff and hard nature of hBN compared to MoS 2 [73]. In both types of filler loaded composites, as the filler concentration is increased beyond 6 wt.% microhardness decreased but is still higher than neat CFREC. This decline in microhardness at the higher filler content might be due to the agglomeration and voids in the composite.

Figure 10: Microhardness versus filler concentration of BN-CFREC and MoS 2 -CFREC.

Analysis of thermal stability and decomposition kinetics Ability of material to maintain its physical integrity and structure properties with minimum deterioration indicates its thermal stability. Composite samples of 10 mg were kept in a crucible of TG analyzer and heated up to 200 °C for 5 minutes to eradicate humidity. Further, the sample was heated to 900 °C at a heating rate of 20 °C min -1 in the presence of N 2 gas to avoid carbon oxidation. The weight loss was monitored as the temperature rise and weight loss as a function of temperature for various composites are plotted as shown in Fig. 11(a-b). The initial early weight loss near 200 °C was due to the evaporation of unreacted monomers and other volatile substances [74]. The volatile evolved could be CO 2 , H 2 O, CO, aliphatic or aromatic compounds [75]. The onset decomposition temperature, transitional temperature T d5% (correspond to 5% weight loss) and T d25% (correspond to 25% weight loss) of the composite were measured and presented in Tab. 6. Fig. 11(a-b) reveals the decomposition of the molecular chain of cross-linked epoxy noticed at 300 to 400 °C. However, depending on the precursor and mode of heat treatment used in the synthesis of carbon fiber, the carbon fibers decompose at high temperature between 400 and 1000 °C as per literature [76]. In the present work polyacrylonitrile (PAN) based carbon fabric is used. Hence the decomposition of fibers starts near at 400 °C as shown in Fig. 11(a-b). The addition of filler to the CFREC upgrades the onset degradation temperature as compared to neat CFREC. The neat CFREC exhibits 25% weight loss at 395 °C, the 4MoS 2 -CFREC and 4BN-CFREC at 430 and 411 °C respectively.

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